1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker's fabric of the on-machine-seamable (OMS.RTM.) variety, such as an OMS.RTM. press fabric for the press section of a papermachine.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, on a moving forming fabric in the forming section of a papermachine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving the fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the fibers in the web to one another to turn the fibrous web into a sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The web, or newly formed paper sheet, itself is directed in a sinuous path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of the drums. The heated drums reduce the water content of the web to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the papermachine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Referring, for the moment, specifically to press fabrics, it should be recalled that, at one time, press fabrics were supplied only in endless form. This is because a newly formed paper sheet is extremely susceptible to marking in the press nip by any nonuniformity in the press fabric or fabrics. An endless, seamless fabric, such as one produced by the process known as endless weaving, has a uniform structure in both its longitudinal (machine) and transverse (cross-machine) directions. A seam, such as a seam which may be used to close the press fabric into endless form during installation on a papermachine, represents a discontinuity in the uniform structure of the press fabric. The use of a seam, then, greatly increases the likelihood that the paper sheet will be marked in the press nip.
In brief, the seam region of any workable on-machine-seamable (OMS.RTM.) press fabric must behave under load, that is, under compression in the press nip or nips, like the rest of the press fabric, and must have the same permeability to water and to air as the rest of the press fabric, in order to prevent the periodic marking of the paper product being manufactured by the seam region. OMS.RTM. is a registered trademark of Albany International Corp.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop an on-machine-seamable (OMS.RTM.) press fabric, because of the comparative ease and safety with which it could be installed on the press section. Ultimately, these obstacles were overcome with the development of press fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A seam is formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less time-consuming to install an OMS.RTM. press fabric, than it is to install an endless press fabric, on a papermachine.
There are several methods for producing a press fabric that can be joined on the papermachine with such a seam. One method is to flat-weave the fabric, in which case the warp yarns are the machine-direction (MD) yarns of the press fabric. To form the seaming loops, the warp ends are woven some distance back into the fabric body in a direction parallel to the warp yarns. Another technique, far more preferable, is a modified form of endless weaving, which normally is used to produce an endless loop of fabric. In modified endless weaving, the weft, or filling, yarns are continuously woven back and forth across the loom, in each passage forming a loop on one of the edges of the fabric being woven by passing around a loop-forming pin. As the weft yarn, or filling yarn, which ultimately becomes the MD yarn in the press fabric, is continuous, the seaming loops obtained in this manner are stronger than any that can be produced by weaving the warp ends back into the ends of a flat-woven fabric. In still another method, a fabric is woven endless, and the endless loop of fabric thereby obtained is flattened and given the form of two fabric layers joined to one another at two widthwise ends of the flattened loop. One or more widthwise yarns are then removed from each of the two widthwise ends to produce a short gap defined by the freed, that is, the newly unwoven portions of, lengthwise yarns at each end. These unwoven portions of the lengthwise yarns are then used as seaming loops when the two widthwise ends are brought together as described above.
Generally, the manufacture of an on-machine-seamable (OMS.RTM.) press fabric includes the attachment of a staple fiber batt to one or both sides thereof. The attachment may be effected by a process called needling (fiber locking) or hydroentangling, while the OMS.RTM. fabric is joined into endless form. Once the desired amount of staple fiber batt has been attached, the loop forming pin or pintle is removed to place the OMS.RTM. press fabric into flat form for shipment and eventual installation on a papermachine. At this time, the staple fiber batt must be cut in the vicinity of the seam to completely separate the two ends of the OMS.RTM. press fabric from one another. Often, the staple fiber batt is cut in a manner that enables it to form a flap over the seaming loops when the OMS.RTM. press fabric is rejoined into endless form. In this way, the seam region is practically indistinguishable from the rest of the paper-supporting side of the press fabric.
On the other side, the "roll" side, of the press fabric, however, some staple fiber batt must be removed from the seaming loops to facilitate the later passage of a pintle therethrough. The removal of this generally small amount of staple fiber batt, nevertheless, makes the seam region slightly more permeable to air and water than the rest of the press fabric. This difference in water permeability, or flow resistance, perhaps ever so slight, is enough to cause sheet marking in some situations.
Several approaches to solve this problem have been taken. One approach involves the use of stuffer yarns with the pintle when the OMS.RTM. press fabric is being joined into endless form on the papermachine.
When fabrics requiring high void volume and water-handling capacity are needed, such as four-layer fabrics, the use of only one seam results in a discontinuity which will mark the paper sheet. The use of two seams in an integrally woven four-layer fabric has been proposed. The two seams are aligned one on top of the other.
In another approach, an OMS.RTM. press fabric comprises two separate on-machine-seamable base fabrics, one fitting inside the loop of the other, laminated to one another during the needling process. The seam regions of the inner and outer base fabrics are offset slightly with respect to one another, so that the seam region of each will coincide with a non-seam region of the other.
After the needling process, the staple fiber batt must be cut in the vicinity of the faceside (paperside) seam to facilitate the separation of the two ends of the OMS.RTM. fabric from one another. On the other side of the faceside seam, some staple fiber must be removed from the seaming loops to facilitate the passage of a pintle therethrough when the seaming loops are meshed together.
Further, some staple fiber must also be removed from the backside of the bottom (rollside) seaming loops to facilitate the passage of a pintle through the seam formed when those seaming loops are meshed together.
All of these approaches are designed to compensate for the differences between the water permeability or flow resistance of the seam region of an OMS.RTM. press fabric and the rest or body of the press fabric. However, none of these approaches have yielded completely satisfactory results for all press types and positions and for all paper grades.
The present invention represents an alternative approach toward solving this problem.